Vitamin D attains biological efficacy subsequent to the liver mediated hydroxylation at carbon 25 and the kidney hydroxylation at carbon 1; resulting in synthesis of 1,25-dihydroxyvitamin D3, the active metabolite of vitamin D. This activation pathway is inhibited by 1,25-dihydroxyvitamin D3, the end-product of the activation scheme. Kidney tubules treated with 1,25-dihydroxyvitamin D3 show a suppression of 25-hydroxyvitamin D3-1-hydroxylase activity and the induction of 25-hydroxyvitamin D3-24-hydroxylase activity. How 1,25-dihydroxyvitamin D3 acts directly on kidney cells to regulate hydroxylase enzyme activity is addressed in this investigation. Chick kidney tubules are used as the in vivo experimental model. Studies address: 1) the fidelity of the action of 1,25-dihydroxyvitamin D3; 2) mediation of the metabolite's response via a transcription-dependent event; 3) subcellular localization of the metabolite in the kidney cell and 4) the involvement of a specific cytosol receptor during expression of the metabolite's action. Results from this study will clarify the mechanism whereby the kidney metabolic activation of 25-hydroxyvitamin D3, i.e., 1-hydroxylation to 1,25-dihydroxy vitamin D3, is suppressed. Such a suppressed activation-scheme may be operational in various metabolic disorders of calcium and phosphorus metabolism, e.g., uremic aspect of renal failure and diabetes associated bone disorders. Once the basis for the action of 1,25-(OH)2D3 to feedback inhibit its own synthesis is understood, then the acquired knowledge will be applied to improving current regimens for preventing and/or treating bone diseases which are associated with lesions in calcium and phosphate metabolism.